This invention relates generally to television tuners and, more particularly, to tuners of the so-called memory fine tuning type wherein the fine tuning range is limited.
Present day television tuners of the rotary type include an incrementally rotatable channel selector for selectively connecting certain ones of a plurality of tuned circuit elements into operative relationship with other tuner elements for each of a plurality of channel selector positions. Because of the difficulty in accurately setting the values of tuned circuit elements, and because of the desirability of accurately tuning the local oscillator for clear reception in each of the channel positions of the station selector shaft, it is customary to include an adjustable impedance device in the oscillator circuit. This impedance device is conventionally adjusted by means of a vernier or fine tuning mechanism selectively connectable to the impedance device by the viewer.
When a memory type fine turning system is employed, a separate impedance device is provided for each of the channel selector positions. The separate impedance devices may be switched into operative relationship with the tuner by means of a wafer switch; or a turret employing a plurality of tuning sticks, each having a separate impedance device, may be used. When such a turret is employed, each of the tuning sticks is sequentially rotated into position for engagement by the tuner to determine the receiving frequency of the tuner.
The tuning range of the impedance device is determined by a number of factors including the expected oscillator drift and, to a large extent, the expected variation in the values of the production components used to manufacture the tuner. As a result, only a fraction of the total tuning range is used to compensate for the oscillator drift, and most of the available tuning range is not used after the set has been shipped from the manufacturer. The unused tuning range can cause problems because it permits excessive detuning of the receiver by the user. Such excessive detuning has been found to be one of the major causes of calls for repair service, particularly when a memory type fine tuning is employed.